Some integrated circuits (ICs) contain field effect transistors (FETs) that have drift regions with a field relief oxide layer thereon that enables higher voltage operation. The drift regions deplete under high drain voltage conditions, allowing the FET to block the voltage applied between the drain and source while supporting conduction during the ON-state of the device. A higher voltage FET tends to be formed with the gate electrode extending over field oxide in order to act as a field plate for the drift region.
LOCal Oxidation of Silicon (LOCOS) is a semiconductor fabrication oxidation process that uses a patterned oxygen diffusion barrier layer, commonly a silicon nitride layer, over areas not meant to be oxidized, where a thermally grown silicon dioxide layer is formed within aperture regions of the oxygen diffusion barrier layer at a given thickness, with a thinner tapered silicon oxide region being formed along the edges of the oxygen diffusion barrier layer. Although the active areas widths bordered by LOCOS oxide can be varied, LOCOS provides a single given oxide thickness across the die and across the wafer.